I. Field of the Invention
This invention relates generally to a device used in pulmonary function testing and more particularly to a device for determining the lung volume of a patient. The device is portable and determines the thoracic gas volume of a patient without enclosing the patient in a sealed chamber. The device further includes a dynamic calibration of the device which is incorporated into the determination of the thoracic gas volume of the patient.
II. Discussion of the Related Art
The lung volume of a patient determines the anatomic and physiologic limits of the patient. These limits may be significant in the prognosis and prescribed treatment of the patient. Over the years, various methods have been used to determine the lung volume and related lung capacity of a patient. These methods have included gas dilution techniques, body plethysmography and radiographic techniques.
The gas dilution techniques include closed-circuit helium dilution, and open-circuit nitrogen washout, either by single or multiple breath techniques. A primary disadvantage of the gas dilution techniques is the inability to measure the lung volume in poorly ventilated airspaces without prolonged rebreathing. Also, gas dilution techniques require maintenance of a leak free connection to the patient, particularly during mechanical ventilatory support. Other disadvantages include difficulty in differentiating leaks in the equipment from mal-distribution, and difficulties related to adequate cleaning of the system.
Past efforts to measure a patient's lung volume using body plethysmography have required that the patient's entire body be enclosed within a sealed chamber. The cabinet interior temperature must be approximately equal to the patient's body temperature before accurate testing may commence. Often times the patient experiences discomfort, anxiety, and a feeling of being trapped within the cabinet. There are other times where a patient is not ambulatory and cannot sit within the sealed cabinet, making the use thereof difficult, if not impossible. Recognizing the need to reduce patient discomfort when using body plethysmography techniques, Andrew C. Jackson, in U.S. Pat. No.5,513,648, discloses a partial body plethysmograph, wherein the patient's head and feet extend out from a sealed enclosure. Even in this system, the patient may experience discomfort, anxiety, and a feeling of being trapped. Thus, there still remains a need for a system and apparatus wherein the lung volume of a patient may be measured without enclosing any portion of the patient in a sealed chamber.
The radiographic techniques mentioned above also have their own shortcomings. Radiographic techniques require radiographs of the patient's lungs to be taken and analyzed, thereby exposing the patient to radiation (either x-ray or nuclear) that may be objectionable to the patient. This technique requires that the patient inspire fully and then hold the inspired breath during exposure. Additionally, oftentimes radiographic instruments are not transportable or immediately available and further, the time required to calculate the lung volume from the radiographs may preclude such use.
Therefore, a need exists for a nonintrusive device that: measures a patient's lung volume, is not affected by the surrounding airspace, does not require an ambulatory patient, and does not require a long time for measurements and calculations.